X‑ray Emission Spectroscopy of Biomimetic Mn Coordination Complexes

Understanding the function of Mn ions in biological and chemical redox catalysis requires precise knowledge of their electronic structure. X-ray emission spectroscopy (XES) is an emerging technique with a growing application to biological and biomimetic systems. Here, we report an improved, cost-effective spectrometer used to analyze two biomimetic coordination compounds, [Mn^IV(OH)₂(Me₂EBC)]²⁺ and [Mn^IV(O)­(OH)­(Me₂EBC)]⁺, the second of which contains a key Mn^IVO structural fragment. Despite having the same formal oxidation state (Mn^IV) and tetradentate ligands, XES spectra from these two compounds demonstrate different electronic structures. Experimental measurements and DFT calculations yield different localized spin densities for the two complexes resulting from Mn^IV–OH conversion to Mn^IVO. The relevance of the observed spectroscopic changes is discussed for applications in analyzing complex biological systems such as photosystem II. A model of the S₃ intermediate state of photosystem II containing a Mn^IVO fragment is compared to recent time-resolved X-ray diffraction data of the same state

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of <i>N</i>,<i>N</i>-bis­(<i>n</i>-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis­(dicarboximide), <b>1</b>, obtained by X-ray diffraction reveals that <b>1</b> has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of <b>1</b> results in a 140 ± 20% yield of triplet excitons (^3*<b>1</b>) in τ_SF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis

Singlet Exciton Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide

The crystal structure of <i>N</i>,<i>N</i>-bis­(<i>n</i>-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis­(dicarboximide), <b>1</b>, obtained by X-ray diffraction reveals that <b>1</b> has a nearly planar perylene core and π–π stacks at a 3.5 Å interplanar distance in well-separated slip-stacked columns. Theory predicts that slip-stacked, π–π-stacked structures should enhance interchromophore electronic coupling and thus favor singlet exciton fission. Photoexcitation of vapor-deposited polycrystalline 188 nm thick films of <b>1</b> results in a 140 ± 20% yield of triplet excitons (^3*<b>1</b>) in τ_SF = 180 ± 10 ps. These results illustrate a design strategy for producing perylenediimide and related rylene derivatives that have the optimized interchromophore electronic interactions which promote high-yield singlet exciton fission for potentially enhancing organic solar cell performance and charge separation in systems for artificial photosynthesis